An inkjet recording apparatus, which operates as an image forming apparatus, carries out printing by discharging ink on a paper recording sheet. The inkjet recording apparatus generally includes an ink cartridge with an ink tank from which the ink is supplied to a print head, and the print head then discharges ink to the sheet.
In such an inkjet recording apparatus, several strategies have been attempted for dealing with a problem of inadequate discharge of the ink, which is caused by air entering into the ink supplying system before the ink is depleted. The strategies have been realized by providing an ink absorbing body or a filter etc.
One example of such strategies can be found in Japanese Laid-Open Patent Application Tokukai 2001-219583/(published on Aug. 14, 2001, hereinafter referred to as Document 1), which discloses an ink cartridge including a filter for capturing air. The filter, whose practical transmission size is 8 μm, is provided in a lower portion of the stream than the ink absorbing body. The ink cartridge also includes a recovering means for applying absorbing pressure, in which the level of the pressure is specified to prevent air from passing through the filter.
Incidentally, the inkjet recording apparatus requires user to change the ink cartridge when the ink cartridge runs out of ink. Thus, the inkjet recording apparatus has to have a function for detecting the remaining amount of ink in the ink cartridge and for informing the user the detection result.
In view of this function, there have been suggested several ink cartridges capable of detecting the remaining amount of ink. One common example of such an ink cartridge uses an optical ink level sensor, which is capable of informing the user that the ink is depleted. This information is provided before the ink supplying system absorbs air. The optical sensor can be provided in a form of electrodes in terms of cost reduction. For example, Japanese Laid-Open Patent Application Tokukaihei 03-288654/1991 (published on Dec. 18, 1991 hereinafter referred to as Document 2) discloses an ink cartridge in which an ink absorbing body (foam material) for absorbing ink is provided inside the ink tank, and an ink supplying path for connecting the ink tank and a print head includes a filter. The ink cartridge has the electrodes in a lower portion in the stream than the filter, i.e., near the discharge end of the ink supplying path, so as to detect if there is any ink remaining in the ink supplying path.
In this inkjet recording apparatus, the ink is supplied from the ink cartridge to the print head via the filter by applying negative pressure with respect to the print head (ink discharging end). Then, depletion of ink in the ink supplying path is detected by checking a current flowing between the electrodes. More specifically, when the remaining amount of ink becomes low in the ink cartridge, there is no ink in the ink supplying path and the current flow stops between the electrodes. Then the cutoff of the current flow between the electrodes is detected as an indication that the ink is depleted.
However, the Document 1 does not mention any strategies for preventing air bubbles from passing through the filter upon discharging of ink.
Further, Document 1 takes no account of the characteristic of ink to be absorbed in the ink absorbing body.
Further, as to Document 2, the structure only accepts an ink absorbing body with an N·R not less than 200, and therefore, the material of the ink absorbing body has to be selected from a limited range.
Further, Document 2 neither takes account of the characteristic of ink to be absorbed in the ink absorbing body. Thus, depending on the type of ink, the inkjet recording apparatus may occur some defects, such as insufficient ink supply when the ink is continuously discharged, or leakage of ink when the ink cartridge is inserted or detached.
Further, when the ink is supplied by applying the negative pressure with respect to the print head (ink discharging end) via the filter, and if the negative pressure excessively increases in the lower stream than the filter in the ink supplying path, air enters into the print head through the end of the nozzle of print head, and may cause inadequate discharge of ink. The increase of the negative pressure may also allow air having been captured by the filter to pass through the filter. The air passed through the filter may block the ink supplying path, or may enter into the print head, thus inducing a risk of inadequate discharge. Further, if the air reaches the ink remaining amount detection section, the current flow between the electrodes stops, and the ink remaining amount detection section may mistakenly judges that the ink is depleted. Accordingly, if the pressure for supplying ink becomes larger than the negative pressure applied to the filter, air enters into the ink supplying path even when there is no decreases of ink remaining amount, thus causing error operation in detecting the remaining amount of ink.
However, the foregoing Documents 1 and 2 do not mention any solutions for such problems.